Recent mobile communication systems use modulation methods, such as code division multiple access (CDMA), quadrature amplitude modulation (QAM), and quadrature phase shift keying (QPSK), in order to make good use of frequency resources.
The above-described conventional modulation methods require the linearity of a power amplifier because of a high peak-to-average power ratio (PAPR) radio-frequency (RF) signals and large envelope fluctuations. However, since the power amplifier operates near a saturation region having a strong nonlinear characteristic in order to maximize the output of power and efficiency, gains and phases of power amplifier signals are distorted.
Therefore, the power amplifier requires an additional linearizer for compensating the nonlinear characteristic of the power amplifier. In order to overcome the nonlinear characteristic of the power amplifier, a feed-forward linearizer and a pre-distortion linearizer have been conventionally proposed.
In the case of a power amplifier using a feed-forward linearizer, signals are dividedly applied to a main path and a sub-path, and carrier signals (or a tone signal and its corresponding signals) on the main path are amplified to a predetermined level by a main amplifier as the power amplifier and then output.
Intermodulation signals of the main amplifier are selectively output by a 3 dB hybrid coupler and attenuated to a predetermined level by an attenuator. The 3 dB hybrid coupler offsets the attenuated signals and signals that are applied to the sub-path and delayed via a first delay loop, so that the intermodulation signals are synthesized.
The resulting signals that are synthesized by the 3 dB hybrid coupler are applied to an error amplifier so that errors of the synthesized signals are corrected and the corrected signals are amplified. Thereafter, the corrected and amplified signals are amplified on the main path and synthesized with signals, which are delayed by a predetermined time via a second delay loop, and output. In the synthesization process, intermodulation distortion (IMD) signals are offset and output.
Meanwhile, in the case of a power amplifier using a pre-distortion linearizer, an applied carrier signal is pre-distorted beforehand by a predetermined pre-distorter. The pre-distorted signal is amplified to a predetermined level by a main amplifier and output. In other words, a pre-distorted signal is generated beforehand and offset by a pre-distorted signal portion of an applied signal, and the remaining portion of the applied signal is amplified and output. In general, the power amplifier using the pre-distortion linearizer can have a small and lightweight structure with a broad bandwidth and a wide operating range at low cost.
A conventional pre-distorter includes separate third- and fifth-order intermodulation signal generators.
FIG. 1 is a circuit construction diagram of a conventional third-order intermodulation signal generator.
Referring to FIG. 1, the third-order intermodulation signal generator includes 90-degree hybrid coupler (i.e., 90-degree directional coupler) 1, a capacitor C, an inductor L, a diode D, and a resistor R, which are interposed between RF input/output signal terminals RFin/RFout.
However, it is difficult to extract only third- and fifth-order intermodulation signal components using the conventional third-order intermodulation signal generator. In order to generate fifth-order intermodulation signals, it is necessary to adopt an additional fifth-order intermodulation signal generator having a different structure from the third-order intermodulation signal generator or an additional complicated circuit.
Also, since the conventional pre-distorter employs intermodulation signal components generated by only one kind of diode D, it is difficult to generate desired third- and fifth-order intermodulation signals.